An adsorption heat pump is one of the most excellent exhaust heat recovery means which can be operated by a low-grade heat energy as a heat source without using any auxiliary power, and is expected to provide a useful means which is applicable to thermal energy utilization systems of environment-favorable type. In the operation of the adsorption heat pump, in order to regenerate an absorbent used therein into which an adsorbate such as water is adsorbed, the adsorbent is heated to desorb the adsorbate therefrom, and then the dried absorbent is cooled to the temperature at which the adsorbent is used for adsorbing the adsorbate again.
Hitherto, absorption-type heat pumps using exhaust heat and warm heat having a relatively high temperature of 120° C. or higher as a heat source for regenerating an adsorbent used therein, have been introduced into thermal and electric energy simultaneous generation plants (cogeneration systems) as a part thereof, and already put into practice. However, in general, exhaust heat and warm heat finally generated in cogeneration equipments and fuel cells have a relatively low temperature not more than 100° C., actually not more than 80° C. Therefore, the heat generated in these equipments is unusable as a heat source for driving the absorption-type heat pumps. In addition, since the low-temperature heat energy generated from these equipments has a low energy density, recovery and reuse of the heat require high costs, so that a substantially whole part thereof is discharged outside without reuse. The total amount of the low-temperature heat energy discharged without reuse reaches 90% or higher of a whole exhaust heat produced, thereby preventing comprehensive improvement in energy utilization efficiency. For this reason, it has been demanded to effectively use the low-temperature exhaust heat, specifically, exhaust heat having a temperature of 100° C. or lower and further from 60 to 80° C.
On the other hand, humidity-control air conditioners such as dehumidifying-type air conditioners and humidifying-type air conditioners are useful as one of exhaust heat recovery regeneration means similarly to the adsorption heat pump. However, there are known no humidity-control air conditioners using the low-temperature heat energy as a driving heat source thereof.
Although the adsorption heat pumps or humidity-control air conditioners are operated by the same principle, adsorbents used therein are required to have different adsorption properties depending upon a temperature of heat sources usable therefor. For example, exhaust heat discharged from gas engine cogeneration systems or solid polymer-type fuel cells which are used as a high-temperature side heat source of the adsorption heat pumps or humidity-control air conditioners, has a temperature of 60 to 80° C. Whereas, in the case where the above exhaust heat is used as the high-temperature side heat source of the adsorption heat pumps or humidity-control air conditioners, the temperature of a cooling side heat source thereof is determined by limitations such as a temperature of location where these equipments are installed. For example, when the equipments are installed in factories or houses, the temperature of the cooling side heat source is a temperature of air outside of the buildings. More specifically, when the adsorption heat pumps or humidity-control air conditioners are installed within the buildings, the operating temperatures thereof are from about 30 to 35° C. on a low-temperature side thereof and from about 60 to 80° C. on a high-temperature side thereof. In the summer season during which need of cold heat is increased, since rise of the outside air temperature is forecast, there is a high possibility that the low-temperature side temperature exceeds the above-specified temperature. Therefore, in order to efficiently utilize the exhaust heat, it is demanded to provide an apparatus which can be operated even under such a condition that a difference in temperature between the low-temperature side heat source and high-temperature heat source thereof is small, and the temperature of the low-temperature side heat source is not less than 30° C. and the temperature of the high-temperature side heat source is not more than 80° C.
To solve the above problem, it is required that an adsorptive material used in the equipments exhibits the following adsorption properties. More specifically, there is demanded such an adsorbent capable of (1) exhibiting an appropriate amount of adsorption even in such a range in which a difference between a relative vapor pressure upon adsorption and that upon desorption is small, (2) having a large difference in amount of adsorption over the range described in the above (1) for the purpose of compactness of the equipments used, and further (3) performing a facilitated desorption even under a high relative vapor pressure.
Various adsorbents have been studied for use in the adsorption heat pumps or humidity-control air conditioners. However, these adsorbents have various problems to be solved.
Y-type zeolites which have been conventionally studied for use as an adsorbent for the adsorption heat pumps or humidity-control air conditioners, adsorb an adsorbate therein even under a relative vapor pressure close to about zero and, therefore, is required to expose to a high temperature ranging from 150 to 200° C. or more in order to bring the relative vapor pressure to substantially zero for desorbing the adsorbate therefrom. Therefore, it may be difficult to apply the Y-type zeolites to the adsorption heat pumps or humidity-control air conditioners utilizing the above low-temperature exhaust heat.
A-type silica gels which have also been conventionally studied as the adsorbent, exhibit insufficient adsorption properties under a low relative vapor pressure. Further, in Japanese Patent Application Laid-Open (KOKAI) No. 9-178292(1997), there is described meso-porous silica (FSM-10, etc.) synthesized by using a micelle structure of surfactant as a template. However, such meso-porous silica is incapable of adsorbing an adsorbate under a low relative vapor pressure. Therefore, there is such a problem that the A-type silica gels or meso-porous silica are inapplicable as a constituting element of the adsorption heat pumps or humidity-control air conditioners utilizing a cooling water obtained from the above cogeneration equipments or fuel cells, or heat obtained from solar energy.
In addition, among the conventional adsorbents, the meso-porous silica tends to suffer from breakage of its structure, in addition to need of improving adsorption properties thereof, and further requires high costs owing to difficult industrial production thereof. On the other hand, the Y-type zeolites or A-type silica gels tend to be insufficient in adsorption properties notwithstanding low production costs and hardly broken structure thereof.
Also, in Japanese Patent Application Laid-Open (KOKAI) No. 11-197439(1999), it is described that a porous aluminophosphate-based zeolite called AlPO-n is used as an adsorbent for dehumidifying-type air conditioners. In Examples of this Japanese Patent Application, there is described the adsorption isotherm of AlPO4-5. However, the zeolite exhibits a slightly high hydrophobic property and, therefore, is incapable of sufficiently adsorbing water vapor at a relative humidity of 0.25 at a temperature of 25° C. More specifically, in the above zeolite, the change in amount of water adsorption thereof when changing a relative humidity by 0.1 in a relative humidity range of from 0.12 to 0.25 at a temperature of 25° C. (which is a necessary condition for the adsorbent of the present invention required for effectively using the low-temperature exhaust heat therein as described hereinlater) is as low as about 0.05 g/g, and, therefore, the above zeolite tends to be deteriorated in such adsorption properties.
In addition, in WO 02/066910, it is described that zeolite containing aluminum, phosphorus and hetero atom is useful as an adsorbent for adsorption heat pumps. However, the invention of WO 02/066910 mainly aims to use automobile exhaust heat having a temperature of about 100° C. which is relatively high among the low-temperature exhaust heats. In the zeolite described concretely in WO 02/066910, the change in amount of water adsorption thereof when changing a relative humidity by 0.1 in a relative humidity range of from 0.12 to 0.25 at a temperature of 25° C. (which is a necessary condition for the adsorbent of the present invention required for effectively using the low-temperature exhaust heat therein as described hereinlater) is as low as about 0.02 g/g, and, therefore, the zeolite tends to be deteriorated in adsorption properties. That is, in the zeolite, the amount of water adsorption thereof at a relative humidity of 0.12 which is an index of the desorption property of the adsorbent used in the present invention as described hereinlater, is large owing to poor desorption property thereof, so that the change in amount of water adsorption of the zeolite tends to become insufficient.
Further, in WO 02/066910, although various atoms including iron are exemplified as the hetero atom, only Si was actually used as the hetero atom in Examples thereof, and the framework density of the zeolite is as low as 16 or less.
In addition, in Examples of Japanese Patent Application Laid-Open (KOKAI) No. 2000-61251, there is described the adsorption isotherm of AlPO—H6. In view of the adsorption properties shown in the adsorption isotherm, the AlPO—H6 satisfies the necessary condition for the adsorbent of the present invention as described hereinlater, i.e., such a condition that the change in amount of water adsorption thereof when changing a relative humidity by 0.1 in a relative humidity range of from 0.12 to 0.25 at a temperature of 25° C. is not less than 0.12 g/g, and, therefore, is considered to provide an adsorbent to which the low-temperature exhaust heat is effectively applicable. However, according to the present inventors' studies, it has been found that the AlPO—H6 tends to be deteriorated in durability.
More specifically, according to the present inventors' studies, zeolites which suffer from structure change between water vapor-adsorbed state and water vapor-desorbed state, are unstable in their structure, so that upon repeated use, the adsorption property thereof tends to be deteriorated, or the structure thereof tends to be broken, thereby rendering the zeolites unusable. The above AlPO—H6 described in Japanese Patent Application Laid-Open (KOKAI) No. 2000-61251 represents a water vapor-adsorbed state, whereas the substance is transformed into a structure represented by AlPO-D when water vapor is desorbed therefrom. This phenomenon is described, for example, in “Molecular Sieve Science and Technology”, Vol. 1 (Springer 1998), p. 160. Thus, the above AlPO—H6 tends to be insufficient in durability when repeatedly used for adsorption and desorption of water vapor.